The present invention relates generally to image generating systems including a reflective type, ferroelectric liquid crystal (FLC) spatial light modulator (SLM). More specifically, the invention relates to an optics arrangement including an FLC compensator cell for allowing the system to generate a substantially continuously viewable image while DC-balancing the FLC material of both the SLM and the compensator cell.
FLC materials may be used to provide a low voltage, low power reflective spatial light modulator due to their switching stability and their high birefringence. However, a problem with FLC materials, and nematic liquid crystal materials, is that the liquid crystal material may degrade over time if the material is subjected to an unbalanced DC electric field for an extended period of time. In order to prevent this degradation, liquid crystal spatial light modulators (SLMs) must be DC field-balanced.
Nematic liquid crystal materials respond to positive or negative voltages in a similar manner regardless of the sign of the voltage. Therefore, nematic liquid crystals are typically switched ON by applying either a positive or negative voltage through the liquid crystal material. Nematic liquid crystal materials are typically switched OFF by not applying any voltage through the material. Because nematic liquid crystal materials respond to voltages of either sign in a similar manner, DC balancing for nematic liquid crystal materials may be accomplished by simply applying an AC signal to create the voltage through the material. The use of an AC signal automatically DC balances the electric field created through the liquid crystal material by regularly reversing the direction of the electric field created through the liquid crystal material at the frequency of the AC signal.
In the case of FLC materials, the materials are switched to one state (i.e. ON) by applying a particular voltage through the material (i.e. +5 VDC) and switched to the other state (i.e. OFF) by applying a different voltage through the material (i.e. xe2x88x925 VDC). Because FLC materials respond differently to positive and negative voltages, they cannot be DC-balanced in situations where it is desired to vary the ratio of ON time to OFF time arbitrarily. Therefore, DC field-balancing for FLC SLMs is most often accomplished by displaying a frame of image data for a certain period of time, and then displaying a frame of the inverse image data for an equal period of time in order to obtain an average DC field of zero for each pixel making up the SLMs.
In the case of an image generating system or display, the image produced by the SLM during the time in which the frame is inverted for purposes of DC field-balancing may not typically be viewed. If the system is viewed during the inverted time without correcting for the inversion of the image, the image would be distorted. In the case in which the image is inverted at a frequency faster than the critical flicker rate of the human eye, the overall image would be completely washed out and all of the pixels would appear to be half on. In the case in which the image is inverted at a frequency slower than the critical clicker rate of the human eye, the viewer would see the image switching between the positive image and the inverted image. Neither of these situations would provide a usable display.
In one approach to solving this problem, the light source used to illuminated the SLM is switched off or directed away from the SLM during the time when the frame is inverted. This type of system is described in copending U.S. patent application Ser. No. 08-361,775, filed Dec. 22, 1994, entitled DC FIELD-BALANCING TECHNIQUE FOR AN ACTIVE MATRIX LIQUID CRYSTAL IMAGE GENERATOR, which is incorporated herein by reference. However, this approach substantially limits the brightness and efficiency of the system. In the case where the magnitude of the electric field during the DC field-balancing and the time when the frame is inverted is equal to the magnitude of the electric field and the time when the frame is viewed, only a maximum of 50% of the light from a given light source may be utilized. This is illustrated in FIG. 1a which is a timing diagram showing the relationship between the switching on and off of the light source and the switching of the SLM image data.
As shown in FIG. 1a, the light source is switched on for a period of time indicated by T1. During this time T1, the SLM is switched to form a desired image. In order to DC balance the SLM, the SLM is switched to form the inverse of the desired image during a time period T2. In order to prevent this inverse image from distorting the desired image, the light source is switched off during the time T2 as shown in FIG. 1a. 
In order to establish a convention to be used throughout this description, the operation of a given pixel 10 of a reflective type FLC SLM using the above mentioned approach of switching off the light source during the time the frame is inverted will be described with reference to FIGS. 1b-d. FIG. 1b shows pixel 10 when it is in its bright state and FIG. 1c shows pixel 10 when it is in its dark state. As illustrated in both FIGS. 1b and 1c, a light source 12 directs light, indicated by arrow 14, into a polarizer 16. Polarizer 16 is arranged to allow, for example, horizontally linearly polorized light, indicated by the reference letter H and by arrow 18, to pass through polarizer 16. However, polarizer 16 blocks any vertically linearly polarized component of the light and thereby directs only horizontally linearly polarized light into pixel 10. This arrangement insures that only horizontally linearly polarized light is used to illuminate pixel 10. For purposes of clarity throughout this description, the various configurations will be described using horizontally linearly polarized light as the initial input light for each of the various configurations.
As also illustrated in FIGS. 1b and 1c, pixel 10 includes a reflective backplane 22 and a layer of FLC material 24 which is supported in front of reflective backplane 22 and which acts as the light modulating medium. The various components would typically be positioned adjacent one another, however, for illustrative purposes, the spacing between the various components is provided. In this example, the FLC material has a thickness and a birefringence which cause the material to act as a quarter wave plate for a given wavelength. In this example, the FLC material is typical of those readily available and has a birefringence of 0.142. Therefore a thickness of 900 nm causes the SLM to act as a quarter wave plate for a wavelength of approximately 510 nm.
FLC material 22 has accompanying alignment layers (not shown) at the surfaces which have a buff axis or alignment axis that controls the alignment of the molecules of the FLC material. For this example of a reflective mode SLM, the SLM is oriented such that the alignment axis is rotated 22.5 degrees relative to the polarization of the horizontally linearly polarized light being directed into the SLM. The FLC also has a tilt angle of 22.5 degrees associated with the average optic axis of the molecules making up the FLC material. Therefore, when FLC material 24 of the pixel is switched to its first state, in this case by applying a +5 VDC electric field across the pixel, the optic axis is rotated to a 45 degree angle relative to the horizontally linearly polarized light. This causes the pixel to act as a quarter wave plate for horizontally linearly polarized light at 510 nm. Alternatively, when the pixel is switched to its second state, in this case by applying a xe2x88x925 VDC electric field across the pixel, the optic axis is rotated to a zero degree angle relative to the horizontally linearly polarized light. This causes the pixel to have no effect on the horizontally linearly polarized light directed into the pixel. In other words, the tilt angle is the angle that the FLC optic axis is rotated one side or the other of the buff axis when the FLC material is switched to its first and second states.
Now that the configuration of the pixel for this example has been described, its effect on the light as it passes through the various elements will be described. Initially, it will be assumed the light is monochrome at the wavelength at which the SLM acts as a quarter wave plate, in this case 510 nm. As illustrated in FIG. 1b, when the FLC material is switched to its first state, which will be referred to hereinafter as its A state, FLC material 24 converts the 510 nm wavelength horizontally linearly polarized light directed into the pixel and indicated by arrow 18 into circularly polarized light indicated by the reference letters C and arrow 26. Reflective backplane 22 reflects this circularly polarized light as indicated by arrow 28 and directing it back into FLC material 24. FLC material 24 again acts on the light converting it from circularly polarized light to vertically linearly polarized light as indicated by reference letter V and arrow 30. The vertically linearly polarized light 30 is directed into an analyzer 32 which is configured to pass vertically linearly polarized light and block horizontally polarized light. Since analyzer 32 is arranged to pass vertically linearly polarized light, this vertically linearly polarized light indicated by arrow 30 passes through analyzer 32 to a viewing area indicated by viewer 34 causing the pixel to appear bright to the viewer.
Alternatively, as illustrated in FIG. 1c, FLC material 24 has no effect on the horizontally linearly polarized light directed into the pixel when the pixel is in its second state, which will be referred to hereinafter as its B state. This is the case regardless of the wavelength of the light. Therefore, the horizontally linearly polarized light passes through FLC material 24 and is reflected by reflective backplane 22 back into FLC material 24. Again, FLC material 24 has no effect on the horizontally linearly polarized light. And finally, since analyzer 32 is arranged to block horizontally linearly polarized light, the horizontally linearly polarized light is prevented from passing through to viewing area 34 causing the pixel to appear dark.
Although the polarization state of the light is relatively straight forward when the light is assumed to be at a wavelength at which the SLM acts as a quarter wave plate, it becomes more complicated when polychromatic light is used. This is because even if the birefringence An of the FLC were constant, the retardance of the SLM in waves would vary with wavelength; furthermore, the birefringence of the FLC material also varies as the wavelength of the light varies. In display applications, this becomes very important due to the desire to provide color displays. FIG. 1d illustrates the effects the SLM has on visible light ranging in wavelength from 400 nm to 700 nm as a function of the wavelength of the light assuming typical FLC birefringence dispersions. Solid line 36 corresponds to the first case when the pixel is in its A state as illustrated in FIG. 1b and the dashed line 38 corresponds to the second case when the pixel is in its B state as illustrated in FIG. 1c. As is illustrated in FIG. 1d, the resulting output of this configuration varies substantially depending on the wavelength of the light as indicated by line 36. In fact, only a little more than 50% of the horizontally linearly polarized light at 400 nm that is directed into the SLM is converted to vertically linearly polarized light using this configuration.
The above described configuration makes use of crossed polarizers. That is, polarizer 16 blocks vertically linearly polarized light and analyzer 32 blocks horizontally linearly polarized light. This means that polarizer 16 and analyzer 32 must be different elements. If both polarizer 16 and analyzer 32 were configured to pass the same polarization of light, they would be referred to as parallel polarizers and could be provided by the same element.
In an alternative system configuration, a polarizing beam splitter may be used to replace both the polarizer and the analyzer. FIGS. 1e and 1f illustrate such a system when pixel 10 is in its A and B states respectively. In this alternative system, light from light source 12 is directed into a polarizing beam splitter (PBS) 40 as indicated by arrow 42. PBS 40 is configured to reflect horizontally linearly polarized light as indicated by arrow 44 and pass vertically linearly polarized light as indicated by arrow 46. The horizontally linearly polarized light indicated by arrow 44 is directed into SLM 24.
When pixel 10 is in its A state as illustrated in FIG. 1e, SLM 24 acts as a quarter wave plate as described above converting the horizontally linearly polarized light to circularly polarized light and reflective backplane 22 reflects this light back into SLM 24. Again, SLM 24 converts this circularly polarized light into vertically linearly polarized light as described above for FIG. 1b and as indicated by arrow 48. Since PBS 40 is configured to pass vertically linearly polarized light, this light passes through PBS 40 into viewing area 34 causing pixel 10 to appear bright.
When pixel 10 is in its B state as illustrated in FIG. 1f, SLM 24 has no effect on the horizontally linearly polorized light. Therefore, the horizontally linearly polarized light that is directed into SLM 24 as indicated by arrow 44 remains horizontally linearly polarized light as it passes through SLM 24, is reflected by backplane 22, and again passes through SLM 24. However, since PBS 40 is configured to reflect horizontally linearly polarized light, this light is reflected back toward light source 12 as indicated by arrow 50 causing pixel 10 to appear dark.
As mentioned above, in the configuration currently being described, the light source is turned off during the time in which the image is inverted for purposes of DC field-balancing the FLC material as illustrated in FIG. 1a. This substantially reduces the brightness or efficiency of the display. In order to overcome this problem of not being able to view the system during the DC field-balancing frame inversion time, compensator cells have been proposed for transmissive SLMs such as those described in U.S. Pat. No. 5,126,864. These compensator cells are intended to correct for the frame inversion during the time when the FLC pixel is being operated in its inverted state. FIG. 2a illustrates a transmissive mode system 200 which includes an SLM 202, a compensator cell 204, a polarizer 206, and an analyzer 208.
As described above for the FLC material of the SLM of the previous configuration, SLM 202 and compensator cell 204 each include an FLC layer which is switchable between an A and a B state. This results in four possible combinations of states for the SLM and compensator cell. For purposes of consistency in comparing various configurations described herein, these four cases will be defined as follows:
Case 1xe2x80x94compensator cell in B state, SLM pixel in A state
Case 2xe2x80x94compensator cell in B state, SLM pixel in B state
Case 3xe2x80x94compensator cell in A state, SLM pixel in B state
Case 4xe2x80x94compensator cell in A state, SLM pixel in A state
For this configuration, Cases 1 and 2 correspond to the normal operation of the system during which the compensator cell is in its B state and the SLM pixels are switched between their A and B states to respectively produce a bright or dark pixel. This is illustrated in the first half of FIG. 2b which is a timing diagram showing the states of the light source, the SLM, and the compensator cell. As shown in FIG. 2b, the light source remains ON throughout the operation of the system. During the first half of the time illustrated in FIG. 2b, the pixels of the SLM are switched between their A and B states to produce a desired image. Cases 3 and 4 correspond to the time during which the frame is inverted for purposes of DC field balancing (i.e. the SLM pixel states must be reversed) and the compensator cell is switched to its A state to compensate for the inversion. This is illustrated by the second half of the diagram of FIG. 2b. To properly DC field-balance the display as well as allow the display to be viewed continuously, Case 1 and Case 3 must give the same results and Case 2 and Case 4 must give the same results. That is, for this configuration, Cases 1 and 3 must both produce a bright pixel and Cases 2 and 4 must both produce a dark pixel.
In this example of a transmissive mode system, both the FLC layer of the SLM pixel and the compensator cell are 1800 nm thick which causes them to act as a half wave plate for a wavelength of 510 nm when in the ON state. In this configuration, the polarizer and analyzer perform the functions performed by polarizer 16 and analyzer 32, or alternatively PBS 40, of the reflective mode systems described above. Polarizer 206 is positioned optically in front of compensator cell 204 and the SLM pixel 202 such that it allows only horizontally linearly polarized light to pass through it into compensator cell 204. Also, analyzer 208 which only allows vertically linearly polarized light to pass through is positioned optically behind SLM 202.
FIGS. 2c and 2d illustrate the net result the above described transmissive system configuration has on light directed in to the system. FIG. 2c shows the results for Case 1 and 2 during which the compensator cell is in its B state and the SLM is switched between its A state for Case 1 and its B state for Case 2. Case 1 is indicated by solid line 210 and Case 2 is indicated by dashed line 212. FIG. 2d shows the results for Case 3 and 4 during which the compensator cell is in its A state and the SLM is switched between its B state for Case 3 and its A state for Case 4. Case 3 is represented by solid line 214 and Case 4 is represented by dashed line 216.
As clearly shown by FIGS. 2c and 2d, this transmissive configuration produces identical results, that is a bright pixel, for Case 1 and 3 as indicated by lines 210 and 214, respectively. It also produces identical results for Cases 2 and 4 as indicated by lines 212 and 216, respectively. It should also be noted that this configuration produces relatively good results over the entire wavelength range from 400 nm to 700 nm. The worst results are at 400 nm where approximately 80% of the horizontally linearly polarized light is converted to vertically polarized light.
Although the compensator cell approach works well for a transmissive SLM as described above, applicant has found that this same general approach does not work as well for a reflective type SLM. To illustrate this difference, and referring to FIG. 3a, a reflective type display system 300 including a reflective type SLM 302 having a reflective backplane 303, a compensator cell 304, a polarizer 306, and an analyzer 308 will be described. Compensator cell 304 is positioned adjacent to SLM 302. As described above for FIGS. 1b and 1c, polarizer 306 is positioned to direct only horizontally linearly polarized light into compensator cell 304. Because the light passes through the SLM and the compensator cell twice in a reflective mode system, the FLC material of SLM 302 and compensator cell 304 are configured to act as quarter wave plates for a wavelength of 510 nm rather than half wave plates as described above for the transmissive system of FIG. 2a. 
In this example, the FLC materials of both SLM 302 and compensator cell 304 are 900 nm thick and both have a tilt angle of 22.5 degrees. The buff axis of the SLM is aligned with the horizontally linearly polarized light directed into the system by polarizer 306. Also, the buff axis of compensator cell 304 is positioned perpendicular to the buff axis of SLM 302. FIGS. 3b and 3c illustrate the net result that system 300 has on light directed in to the system. FIG. 3b shows the results for Case 1 and 2 during which the compensator cell is in its B state and the SLM is switched between its A state for Case 1 and its B state for Case 2. Case 1 is indicated by solid line 310 and Case 2 is indicated by dashed line 312. FIG. 3c shows the results for Case 3 and 4 during which the compensator cell is in its A state and the SLM is switched between its B state for Case 3 and its A state for Case 4. Case 3 is represented by solid line 314 and Case 4 is represented by dashed line 316.
As clearly shown by FIGS. 3b and 3c, system 300 produces identical results, that is, a bright pixel for Case 1 and 3 as indicated by lines 310 and 314, respectively. It also produces identical results for Cases 2 and 4 as indicated by lines 312 and 316, respectively. However, this configuration does not produces very good results over the entire wavelength range from 400 nm to 700 nm. The worst results are at 400 nm where only approximately 5% of the horizontally linearly polarized light is converted to vertically polarized light. At a wavelength of about 500 nm about 50% of the horizontally linearly polarized light is converted to vertically linearly polarized light. The best results are at 700 nm where about 80% of the horizontally linearly polarized light is converted to vertically linearly polarized light. Since the point to adding the compensator cell is to increase the efficiency or brightness of the system, this arrangement does not improve the efficiency or brightness for the lower wavelength range when compared to the system of FIG. 1b and 1c which simply turns OFF the light source during the DC field-balancing time.
As can be clearly seen when comparing FIGS. 3b-c to FIGS. 2c-d, the effects on the light caused by the various components of the reflective configuration of FIG. 3a are very much different from the effects on the light caused by the transmissive configuration of FIG. 2a. That is, the reflective configuration of FIG. 3a is not optically equivalent to the transmissive configuration of FIG. 2a even though it may initially seem as though they should be optically equivalent. These two configurations are optically different from one another because the light must pass through the SLM and compensator cell twice in the reflective configuration with the first pass through the compensator being before the two passes through the SLM and the second pass through the compensator cell being after the two passes through the SLM.
Due to this difference in the transmissive and reflective configurations, it has proved difficult to provide a reflective type system which is DC field-balanced and is substantially continuously viewable while providing improved efficiency or brightness compared to a system which simply turns off the light source during the DC field-balancing portion of the frame. The present invention provides arrangements and methods for overcome this problem.
As will be described in more detail hereinafter, a reflection mode, spatial light modulating system and methods of operating the system are herein disclosed. The reflection mode, ferroelectric liquid crystal spatial light modulating system, includes a light reflecting type spatial light modulator. The spatial light modulator has a light reflecting surface cooperating with a layer of ferroelectric liquid crystal light modulating medium switchable between first and second states so as to act on light in different first and second ways, respectively. A switching arrangement switches the liquid crystal light modulating medium between the first and second states and an illumination arrangement produces a source of light. An optics arrangement is optically coupled the spatial light modulator and the illumination arrangement such that light is directed from the source of light into the spatial light modulator for reflection back out of the modulator and such that reflected light is directed from the spatial light modulator into a predetermined viewing area. A compensator cell is also positioned in the optical path between the light source and the viewing area. The compensator cell has a layer of ferroelectric liquid crystal light modulating medium switchable between a primary and a secondary state so as to act on light in different primary and secondary ways, respectively.
In one embodiment, the optics arrangement includes a passive quarter wave plate positioned in the optical path between the light source and the spatial light modulator and in the optical path between the spatial light modulator and the viewing area. In this embodiment, the compensator cell is positioned in the optical path between the light source and the spatial light modulator and in the optical path between the spatial light modulator and the viewing area.